October 31, 2025, ©. Leeham News: We do a series about ideas on how the long development times for large airliners can be shortened. New projects talk about cutting development time and reaching certification and production faster than previous projects.

The series will discuss the typical development cycles for an FAA Part 25 aircraft, called a transport category aircraft, and what different ideas there are to reduce the development times. We will use the Gantt plan in Figure 1 as a base for our discussions.

We have exited the Preliminary Design phase through the Preliminary Design Reviews, PDRs, and now enter the Detailed Design phase.

Figure 1. A generic new Part 25 airliner development plan. Source: Leeham Co. Click to see better.

Overview of the Detailed Design Phase

Now that we have requirements, architecture, interfaces, and high-level solutions defined, it is time for the team and suppliers to go into the details for every piece of equipment and structural components.

It is also time for the team to negotiate with certification authorities on how the OEM will show compliance to relevant regulations in detail.  (We will save the regulatory compliance planning topic for a future article.)

From the development program perspective, the spending continues to rise.  Many engineering work hours are required to release documents and datasets.  (Note: Our Gantt chart shows thousands of participants, making the engineering time consumption in this phase very costly.)

Suppliers may also start spending money to prepare for manufacturing.  By the end of this phase, the aircraft configuration should be frozen so that partners can deliver the right parts for qualification and flight test in the next phase.  Any major change in direction after this phase will be very costly.

Product Development

The structures teams have a lot of head-down work to do during this phase.  They need to develop the full structural solution down to every part, nuts and bolts.  They also need to optimize the design.  We will examine some of their activities in the next article.

Systems teams continue to traverse to the bottom of the “V”, Figure 2.   Engineers need to conduct analyses such as Fault Tree Analysis and Common Mode Analysis.   These teams need to ensure that suppliers are following appropriate processes when designing software and hardware for the aircraft as well.

Figure2. The system development V. Source: Wikipedia.

The integration team closely monitors key parameters such as weight, drag, etc.  These parameters have a direct impact on the aircraft’s performance.  If some of these parameters are trending in the wrong direction, the team may need to work with suppliers to address specific issues or execute risk mitigation plans.

Engineering tests also begin during this phase.  As mentioned in Part 11, a seat supplier may conduct crashworthiness tests to de-risk the seat design.  Systems teams may also kickoff hardware-in-the-loop tests with prototype software and hardware to derisk their systems.  These test-beds may include iron birds or integration test rigs among others.  They are essentially an aircraft without the major structures, designed to test highly complex/integrated systems and their interfaces.  They can even allow human-in-the-loop testing.  These tests are not intended for certification credits.  They are meant for derisking the design through early discovery.

In parallel, design teams must start drafting qualification plans – based on agreements with authorities – to ensure that parts can meet design specifications.  Even though qualification tests will be carried out during the next phase, their preparation begins in this phase.

For instance, the aircraft OEM would ask a supplier to demonstrate that a horizontal stabilizer actuator can operate and survive in the environmental conditions inside the tail so that aircraft OEM can show compliance to airworthiness standards.  The team needs to define test conditions, test articles, test apparatus, and pass/fail criteria, and prepare test documents.

Certain qualified or authorized personnel may also be required to witness the conformity inspection, test setup, and the test itself.  Fortunately, some of these test conditions, procedures, and criteria are well documented in authority accepted industry standards such as the RTCA DO-160, Environmental Conditions and Test Procedures for Airborne Equipment.

Other tests, such as durability and repairability tests for the airframe, require more negotiations with authorities.

Instructions for Continued Airworthiness

Although the aircraft is still being designed, some of the continued airworthiness related activities can kick off in this phase.  Maintenance planning is one example.  During the detailed design phase, the OEM’s maintenance engineering and certification teams would initiate the maintenance review board process with the certification authority, Figure 3.

Figure 3. The Maintenance Review Board process. Source: FAA AC 121-22C. Click to see better.

The objective is to develop scheduled maintenance tasks and intervals acceptable to regulators, operators, and the aircraft OEM.  This process focuses on Maintenance Significant Items (MSIs).  An MSI is an item where the failure or malfunction of it could affect safety (on the ground or in the air), be undetectable during operation, or have a significant operational or economic impact.  The board can also request an examination of items based on lessons learned.

At a high level, the maintenance review board (MRB), led by the certification authority, is formed during this phase to enable the maintenance review board process.   The MRB works with the aircraft OEM to establish the Industry Steering Group (ISG) as an intermediary group, consisting of participants from the regulatory authorities, the aircraft OEM, suppliers, and operators, to propose minimum scheduled maintenance requirements to the MRB when the work is completed.

This committee defines Working Groups (WGs), appoints specialists to conduct the required work, and manages these groups.  Members of each WG then analyze the Maintenance Significant Items along with supporting data from the aircraft OEM to develop scheduled maintenance tasks and intervals using the Maintenance Steering Group 3 (MSG-3) process.  WG #1, #2, and #3, as an example, may conduct the initial maintenance tasking/scheduling work on mechanical systems, electrical systems, and aircraft structures.  These WGs could progress more or less in parallel, especially when there is no resource conflict.  WG #4 may bring outstanding items from WG #1, #2, and #3 for additional analyses, validations, and verifications.  If required, additional WGs can be added.

Since WG members require information related to the final design, they usually start work when there is a sufficient amount of information available.  Specialists may need, as an example, mature fault trees to discern the effects of failures.  They may also need data from system safety analysis and reliability analyses.  The team will spend the next year or so to fully develop their proposals.

Once analyses are completed, WG members present their findings and proposals to the ISC.  These proposals are then reviewed and accepted by the committee.  The accepted tasks/intervals are compiled into a Maintenance Review Board Report (MRBR) proposal.  The proposal will then be submitted to the MRB for approval.  Once approved, the aircraft OEM can provide the maintenance planning document and the Maintenance Review Board Report to operators so that they can develop their maintenance plan.  Here is a high-level example of a Maintenance Significant Item (MSI) and its result (note: we skipped the detailed analysis):

• MSI: Flap position indication
• Function: Provide accurate flap position indication to support safe takeoff, approach, and landing
• Functional failures:
  1. No indication (evident)
  2. Indication inaccurate beyond tolerance (hidden)
  3. Rigging out of tolerance (hidden)
• Sample tasks & intervals
  1. Functional check (for accuracy) – verify indication within tolerance at each detent. Initial intervals: __ flight-hours / __ flight cycles.
  2. Detailed Inspection (rigging, linkages) – backlash, stops, sensor arms. Intervals: __ flight-hours / __ flight cycles.
  3. Restore/calibrate – if a functional check failed or limit exceeded. Interval: Condition-based trigger by functional check.

The initial process typically doesn’t end until sometime during the flight test.  WGs often use data from rig tests and flight tests to support their analyses.  These groups sometimes use rigs or flight test aircraft to check their findings or proposals as well.

The MRB process actually continues after type certification.  Hence, there are opportunities to increase maintenance intervals to lower maintenance costs as the fleet matures.

Manufacturing

Manufacturing engineers had already started working on tooling design in the last phase.  Since a lot of details were not available at that time, they would have worked on the concept and attempted to generate some details wherever they could.

In this phase, Manufacturing engineers need to complete the tooling design.  They need to work closely with the design team to ensure detailed datasets are released in a proper sequence so that the manufacturing team can update their design and order long lead-time items from suppliers.  If automation equipment is required for the assembly line, the manufacturing team may even start testing this equipment at the supplier site before installing them on the final line at a later time.

Additional Work for Start-Ups

Start-ups have quite a bit of extra work to do during this phase.  A mature aircraft OEM already has the IT infrastructure to support its customers and to comply with type certificate holder obligations.  Start-ups, on the other hand, probably have not paid much attention to these matters up to now.  For instance, §21.3 requires a type certificate holder to report failures, malfunctions, and defects.  Obviously, a start-up doesn’t hold a type certificate.  Yet, the company may want to use rig tests and flight tests as test cases to mature the reporting system.  This means that the company should implement the reporting system during this phase to allow deployment before major test campaigns begin.

A mature aircraft OEM also has an established flight test department.  A start-up, on the other hand, would need to create one from scratch.  The company needs to identify roles and responsibilities of the flight test organization, create a safety and risk management system, develop operational plans, establish crew qualification, etc.  The company also needs to start staffing the organization.  These are not super difficult tasks, but money, time, and resources are still needed to complete these activities.

Speeding Up the Detailed Design Phase

The Detailed Design phase is a labor-intensive phase.  There is a lot of data creation, which also means there are a lot of reviews and approvals to support data releases.  Engineers can use AI as a complementary tool to conduct quality checks for documents.  This could reduce rework, leading to a shorter release cycle time.

Proper progress reporting can also help maintain the schedule.  A burn-down chart is an easy way to understand progress.  Yet, sometimes the last 20% of the work could take 80% of the time.  Other times, team members might release a low-quality dataset to show progress, but end up reworking the product in a few weeks.  Project managers need to find transparent and meaningful ways to report progress to avoid surprises.  Minimizing rework is crucial for maintaining a schedule.

Rigorous control of changes is paramount.  The team must understand the upstream and downstream impact of a change.  They also need to evaluate the necessity of a change.  A scope creep at this stage could be detrimental to the program because every team is trying to release all the necessary data to enable the production of conforming articles for various tests.  A change that could affect multiple teams must be carefully evaluated.